The present invention relates in general to ultrasonic aspirators and in particular to an ultrasonic aspirator intended for use in surgical operations for removing tissue. More specifically, the ultrasonic aspirator is of the kind including an integrally implemented amplitude transformer with a tip intended to be brought into contact with the tissue, and to serve as a kind of knife with the ability to remove tissue with precision, the tissue being comminuted by the ultrasonic effect into fragments which are sucked away, aspirated, through the elongate tip portion of the ultrasonic aspirator, this portion being the one providing the removal mentioned. Somewhat improperly the ultrasonic aspirator is hereinafter called an ultrasonic knife. A transducer working as a vibrator is mechanically connected to the amplitude transformer and causes the tip portion to oscillate longitudinally. A frequency generator feeds electrical energy of ultrasonic frequency to the transducer unit, which converts this energy into a mechanical longitudinal oscillation, the amplitude of which is reinforced by the amplitude transformer so that the tip portion executes the longitudinal oscillation with great oscillation amplitude.
Ultrasonic knives of the kind mentioned above are known from such as U.S. Pat. Nos. 3,526,219, 4,063,557, 4,223,676, 4,525,115 and EP-A- 0 139 753.
In order that an ultrasonic knife shall function clinically satisfactorily it is an absolute requirement that the tip portion executes a pure longitudinal oscillation movement. Oscillation in the transverse direction must not occur, since such movement would be directly destructive for operations in sensitive areas. Every osciallating mass, and particularly a complicated ultrasonic transducer, has a plurality of resonance frequencies at which oscillation amplitudes in the transverse and/or longitudinal direction are possible. The resonance frequencies change when the resonance conditions of the oscillating body are modified, e.g. when it is immersed in water or comes into contact with tissue. What is particularly important for an ultrasonic knife is that when the knife is in operation, and thereby changes its resonance frequency, the frequency generator must change the frequency of the electrical energy supplied to the knife so that the knife continues to oscillate and thereby to remove material. It is known to use frequency control circuits for this purpose, and voltage controlled oscillators in particular. It is then important that the voltage controlled oscillator locks on to the resonance frequency pertaining to the longitudinal oscillation of the ultrasonic converter and not to its harmonics, and neither to any other resonance frequency pertaining to transverse oscillations. The undesired resonance frequencies are denoted spurious frequencies below. Depending on the ultrasonic transformer geometry, the resonance frequencies, including the spurious resonance frequencies, may be close to each other with regard to frequency, and different means have therefore been used to ensure that the voltage controlled oscillator really locks on to the desired resonance frequency. Accordingly, there is described in the U.S. Pat. No. 4,275,363 a frequency sweeping means which first sweeps a frequency generator over a wide frequency range before the knife is allowed to start, this range being much wider than the one covered by the frequency-following oscillator used, thus to determine the frequency at which the desired resonance (longitudinal oscillation at greatest amplitude) occurs and thereafter to adjust the frequency generator such that the oscillator locks on to this desired frequency. Apart from a voltage controlled oscillator the means includes a resonance point detector, sweeping circuits, phase detectors, flipflops, memory etc. After the desired frequency has been set, the knife may be used for cutting. U.S. Pat. No. 4,223,767 shows a frequency control circuit where a fixed filter has transmission properties such that it only transmits frequency control signals corresponding to a frequency close to the desired resonance frequency.
Two control loops, one for frequency control and the other for amplitude control, are used. The stroke of the knife tip is used as control variable. The total impedance of the resonating system actuates the phase shift of an amplifier and controls the frequency in the frequency control loop. In this patent the physical length of the amplitude transformer is half the wave length of the desired resonance frequency.
The present invention has the object of achieving a simplified frequency control circuit which automatically follows the resonance frequency alterations of the knife and securely locks the frequency generator oscillator on to the desired resonance frequency (longitudinal oscillation with intended amplitude). The electric circuit used here must function securely, have a simple construction, simple tuning and be able to produced using standard components. This is achieved in accordance with the invention in that the amplitude transformer is given definite geometric forms, which are described in detail below, resulting in that the spurious resonance frequencies are greatly separated with respect to frequency from the desired resonance frequency. With reference to the desired resonance frequency the frequency separation is in the order of magnitude of about 20%. In this way, a simple conventional oscillator with a narrow adjustment range can be used without risk of the oscillator locking on to incorrect frequencies. The phase angle between current and voltage of the electrical energy supplied to the knife is used as control variable.
The mentioned geometrical form in accordance with the invention of the ultrasonic transformer furthermore results in the advantage that the ultrasonic knife may be used repeatedly without needing to be exchanged after each operation, as is the case with the knife according to the U.S. Pat. No. 4,223,676, for example. The inventive ultrasonic knife will thus be cheap in operation. In addition, it may be sterilized by autoclaving, which is an advantage over the knife according to the U.S. patent just mentioned, which must be sterilized in the presence of ethylene oxide gas. The ultrasonic knife in accordance with the invention can sustain the high autoclave temperature due to the material selected for the amplitude transformer and the transducer and due to the prestressing of the transducer as described further below and will not disintegrate.
The geometrical form in accordance with the invention also entails that problems relating to so called parametric pumping are reduced. In short parametric pumping means that non-linear phenomenons appearing in the material structure, especially in a resonance body which is excited by energies of a spectrum of different frequencies which are generated, due to cavitation phenomenons, will develop a number of non-desired resonances in the resonance body.
In accordance with the invention the amplitude transformer shall have a length in the order of magnitude 0.72 times the wave length of the desired resonance frequency. In addition, taken from the connection to the transducer and to the tip of the knife, it shall be made in a single unit comprising three sections, namely a cylindrical section nearest the transducer, followed by a section the geometry of which that of a wave function of the fourth order Fourier form and an outmost, substantially conical tip portion. This geometry gives a very well-defined longitudinal oscillating movement for the tip portion simultaneously as the mechanical stresses on the tip are distributed along a portion of the tip length, this portion being a distance inwards of the tip. The life of the knife is thus increased substantially.
The geometrical form of the amplitude transformer is intermediately between an exponential function and a wave function of the fourth order Fourier form.
The invention will now be described in detail with reference to the accompanying drawings, on which